Title: Small molecules that regulate proteasome activity The ubiquitin-proteasome system plays an important role in regulating the cell cycle, cancer growth, and metastasis. Activation of the proteasome is a critical process that is required for optimal proteolytic activity. Inhibitors of the proteasome were shown to arrest tumor growth, tumor spread, and angiogenesis. On the other hand, drug-like proteasome activators are rare, and their biomedical applications remain to be determined. Although there is tremendous interest in developing drugs targeting the proteasome, currently there is only one clinically available proteasome inhibitor, Bortezomib, for anti-cancer therapy. The goal of this study is to identify and synthesize small molecules that can specifically and potently inhibit or activate the proteasome. Our preliminary results indicated that terpenoid natural products, such as betulinic acid, activated proteasome, and chemical modifications on betulinic acid turned it into proteasome inhibitors. We hypothesize that potent proteasome activators or inhibitors could be identified from terpenoid natural products and their potency can be further increased by chemical modifications. The goal will be achieved and the hypothesis will be tested by carrying out the following specific aims: 1) to discover new terpenoids that activate or inhibit the proteasome; 2) to obtain a next generation of potent proteasome inhibitors through lead optimization; 3) to determine the drug binding site and mode of action of the terpenoid proteasome inhibitors. We have established a panel of terpenoids and the strategies for lead optimization to achieve our goal in the discovery of potent proteasome activators and inhibitors that can regulate the proteasome activity at low nanomolar or sub-nanomolar concentrations. It is expected that a new class of novel proteasome regulators (activators and inhibitors) and the molecular target of the inhibitor will be identified upon completion of the proposed studies. Moreover, the study will also determine the therapeutic potential of the proposed proteasome regulators for diseases such as cancers and Huntington's disease. Discovery of novel proteasome activators and inhibitors would not only provide a very useful tool for dissecting the molecular mechanisms of proteasome-mediated cellular functions, but would also have the potential to be developed as therapeutics to treat diseases such as cancers.